There is a continuous development of new generations of mobile communications technologies to cope with increasing requirements of higher data rates, improved efficiency and lower costs. High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), together referred to as High Speed Packet Access (HSPA), are mobile communication protocols that were developed to cope with higher data rates than original Wideband Code Division Multiple Access (WCDMA) protocols were capable of. The 3rd Generation Partnership Project (3GPP) is a standards-developing organization that is continuing its work of evolving HSPA and creating new standards that allow for even higher data rates and improved functionality.
In a radio access network implementing HSPA, a user equipment (UE) is wirelessly connected to a radio base station (RBS) commonly referred to as a NodeB (NB). A radio base station is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
3GPP has evaluated the potential benefits of uplink transmit (Tx) diversity in the context of HSUPA. With uplink transmit diversity, UEs that are equipped with two or more transmit antennas are capable of utilizing all of them for uplink transmissions. This is achieved by multiplying a UE output signal with a set of complex pre-coding weights, a so-called pre-coding vector with one pre-coding weight for each physical transmit antenna. The rationale behind uplink transmit diversity is to adapt the pre-coding weights so that user and network performance is maximized. Depending on UE implementation the antenna pre-coding weights may be associated with different constraints. Within 3GPP two classes of transmit diversity are considered:                Switched antenna transmit diversity, where the UE at any given e instance transmits from one of its antennas only.        Beamforming where the UE at a given time-instance can transmit from more than one antenna simultaneously. By means of beamforming it is possible to shape an overall antenna beam in the direction of a target receiver.        
During 2009 and 2010 the 3GPP evaluated the merits of open loop beam forming and open loop antenna switching for uplink transmissions in WCDMA/HSPA. These techniques are based on that UEs equipped with multiple transmit antennas exploit existing feedback e.g. feedback transmitted on the Fractional Dedicated Physical Channel (F-DPCH) or on the E-DCH HARQ Acknowledgement Indicator Channel (E-HIGH) to determine a suitable pre-coding vector in an autonomous fashion. The purpose of pre-coding the signals is to “maximize” the signal to interference ratio (SIR) at the receiving NodeB. Since the network is unaware of the applied pre-coding weights the NodeBs will experience a discontinuity in the measured power whenever a change in pre-coding weights occurs. A summary of the 3GPP studies on open loop transmit diversity techniques can be found in 3GPP's technical report TR 25.863, UTRA: Uplink Transmit Diversity for High Speed Packet Access.
Recently there have been proposals for introducing closed loop transmit diversity for WCDMA/HSPA. Closed loop transmit diversity refers to both closed loop beam forming and closed loop antenna switching. At the 3GPP meeting RAN #50 a work item with the purpose of specifying support for closed loop transmit diversity was agreed. Contrary to the open loop techniques where the UE decides pre-coding weights autonomously, closed loop techniques are based on that the network, e.g., the serving NodeB, selects the pre-coding vector with which the signal is multiplied. In order to signal the necessary feedback information from the network to the UE, the NodeB can either rely on one of the existing physical channels, e.g., F-DPCH, or a new feedback channel could be introduced.
Uplink multiple-input-multiple-output (MIMO) transmission is another related technique that has been proposed as a candidate for WCDMA/HSPA in 3GPP standard release 11. A study item on uplink MIMO for WCDMA/HSUPA was started at the 3GPP RAN #50 plenary meeting. For uplink MIMO, different data is transmitted from different virtual antennas in so-called streams. Each virtual antenna corresponds to a different pre-coding vector. Note that closed loop beam forming can be viewed as a special case of uplink MIMO where no data is scheduled on one of the possible virtual antennas.
MIMO technology is mainly beneficial in situations where the “composite channel” is strong and has high rank. The term composite channel includes the potential effects of transmit antenna(s), PAs, as well as the radio channel between the transmitting and receiving antennas. The rank of the composite channel depends on the number of uncorrelated paths between the transmitter and the receiver. Single-stream transmissions, i.e. beam forming techniques, are generally preferred over MIMO transmissions in situations where the rank of the composite channel is low e.g. where there is a limited amount of multi-path propagation and cross polarized antennas are not used, and/or the path gain between the UE and the NodeB is weak. This results from a combined effect of that the theoretical gains of MIMO transmissions is marginal at low SIR operating point and that inter-stream interference can be avoided in case of single-stream transmissions.
For HSDPA with MIMO, which was introduced in Rel-7 of the 3GPP standard, there is a High-Speed (HS) code reuse between transmissions from two different virtual antennas. More specifically, for downlink operation it was specified in the 3GPP standard that the transmissions from the two virtual antennas—i.e. the two streams—should use exactly the same spreading codes. To a large extent, this was motivated by the fact that the HS codes are a resource that is shared amongst all UEs served by a particular cell. While the benefits with this approach are that it provides an economic use of the available HS codes and also reduce the receiver complexity, a drawback is that the inter-stream interference becomes an issue for dual-stream transmission in the downlink since the different streams utilize the same spreading codes, i.e. ortogonality between the transmissions on the different streams is only provided via the pre-coder and not by means of using different channelization codes.
Currently HSUPA does not allow MIMO transmission since only single stream transmissions are allowed in the uplink. However, if uplink MIMO is to be introduced for WCDMA/HSPA in 3GPP standard release 11, a scheme for allocation of channelisation codes to different UL MIMO streams should be predefined in the 3GPP standard. It is desirable that such a scheme for allocation of channelisation codes is particularly adapted to uplink transmissions and advantageously exploits differences between uplink and downlink transmissions.
Hereinafter, the term code allocation will be used to refer to allocation of channelisation codes. Channelisation codes may synonymously also be referred to as spreading codes. The term HS code allocation means High-Speed Physical Downlink Shared Channel (HS-PDSCH) code allocation in a HSUPA transmission.